Recently, the world’s first room-temperature maser was demonstrated. The maser consisted of a sapphire ring housing a crystal of pentacene-doped p-terphenyl, pumped by a pulsed rhodamine-dye laser. Stimulated emission of microwaves was aided by the high quality factor and small magnetic mode volume of the maser cavity yet the peak optical pumping power was 1.4 kW. Here we report dramatic miniaturization and 2 orders of magnitude reduction in optical pumping power for a room-temperature maser by coupling a strontium titanate resonator with the spin-polarized population inversion provided by triplet states in an optically excited pentacene-doped p-terphenyl crystal. We observe maser emission in a thimble-sized resonator using a xenon flash lamp as an optical pump source with peak optical power of 70 W. This is a significant step towards the goal of continuous maser operation.

The strong coupling regime is essential for efficient transfer of excitations between states in different quantum systems on timescales shorter than their lifetimes. The coupling of single spins to microwave photons is very weak but can be enhanced by increasing the local density of states by reducing the magnetic mode volume of the cavity. In practice, it is difficult to achieve both small cavity mode volume and low cavity decay rate, so superconducting metals are often employed at cryogenic temperatures. For an ensembles of N spins, the spin–photon coupling can be enhanced by $$\sqrt N$$N through collective spin excitations known as Dicke states. For sufficiently large N the collective spin–photon coupling can exceed both the spin decoherence and cavity decay rates, making the strong-coupling regime accessible. Here we demonstrate strong coupling and cavity quantum electrodynamics in a solid-state system at room-temperature. We generate an inverted spin-ensemble with N ~ 10¹⁵ by photo-exciting pentacene molecules into spin-triplet states with spin dephasing time $$T_2^*\sim 3$$T 2 * ~ 3 μs. When coupled to a 1.45 GHz TE_01δ mode supported by a high Purcell factor strontium titanate dielectric cavity ( $$V_{\mathrm{m}}\sim 0.25$$V m ~ 0.25 cm³, Q ~ 8,500), we observe Rabi oscillations in the microwave emission from collective Dicke states and a 1.8 MHz normal-mode splitting of the resultant collective spin–photon polariton. We also observe a cavity protection effect at the onset of the strong-coupling regime which decreases the polariton decay rate as the collective coupling increases.

The multiplexing capacity of conventional fluorescence materials are significantly limited by spectral overlap and background interference, mainly due to their short-lived fluorescence lifetimes. Here, we adopt a novel Gd³⁺ doping strategy in NaYF₄ host materials, realized tuning of upconversion photoluminescence (UCPL) lifetimes at selective emissions. Time-correlated single-photon counting (TCSPC), was applied to measure the photoluminescence lifetimes accurately. We demonstrated the large dynamic range of lifetimes of upconversion nanoparticles with good upconversion quantum yields, mainly owing to the dominance of high efficient energy transfer upconversion mechanism. The exceptional tunable properties of upconversion materials allow great potential for them to be utilized in biotechnology and life sciences.

A luminescent concentrator functioning as a bright source of yellow light is reported. It comprises a waveguide made of cerium-doped YAG crystal, in the form of a long-thin rectangular strip, surrounded by flowing air and optically pumped from both sides with blue light from arrays of high-efficiency InGaN LEDs. Phosphor-converted yellow light, generated within the strip, is guided to a glass taper that is butt-coupled to one of the strip’s end faces. Up to 20 W of optical power, centered on 575 nm with a linewidth of 76 nm, can be continuously radiated into air from the taper’s 1.67 mm × 1.67 mm square output aperture. The intensity of the outputted light is significantly greater than what any yellow (AlGaInP) LED can directly produce (either singly or arrayed), with only a modest increase in linewidth. Furthermore, the wall-plug efficiency of the source exceeds that of any yellow laser. The concept allows for further substantial increases in intensity, total output power and wall-plug efficiency through scaling-up and engineering refinements.